[Haskell-cafe] Type error

[Alfonso Acosta]

Thanks for your answer, The functional dependencies solution doesn't
help because my instances cannot satisfy them (there are various
DestPort and SourcePort instances with are required to support various
HDSignals). I tried with existentials, but I'm still getting an error
(I'm not that comfortable working with existentials yet :)).

Example.hs:24:41:
    Couldn't match expected type `forall a.
                                  (HDPrimType a) =>
                                  HDSignal a -> b'
           against inferred type `(forall a. (HDPrimType a) => HDSignal a)
                                  -> d'
    In the second argument of `($)', namely `(push2 supplySig) dix d'
    In the expression: (plugSig six s) $ ((push2 supplySig) dix d)
    In the definition of `connectIx':
        connectIx six s dix d = (plugSig six s) $ ((push2 supplySig) dix d)


Here is the code

module Example where


data HDSignal a = HDSignal
class HDPrimType a where
class PortIndex  a where

class SourcePort s where
 -- Plug an external signal to the port
 plugSig  :: (HDPrimType a, PortIndex ix) => ix -> s  -> (forall
a.HDPrimType a => HDSignal a -> b) -> b

class DestPort d where
 -- Supply a signal to the port
 supplySig  :: (PortIndex ix, HDPrimType a) => (forall a. HDPrimType a
=> HDSignal a) -> ix -> d -> d


-- Connect providing indexes
connectIx :: (SourcePort s, PortIndex six, DestPort d, PortIndex dix) =>
             six -> s -> dix -> d -> d
-- This can seem ugly,
-- it would be easier for us having different types for plugSig and supplySig
-- but the final user would find it much more difficult to deal with
-- supplysig
connectIx six s dix d = plugSig six s $ (push2 supplySig) dix d


push2 :: (a -> b -> c -> d) -> b -> c -> a -> d
push2 f = (\b c a -> f a b c)



On 4/6/07, Chris Kuklewicz <haskell at list.mightyreason.com> wrote:
> I'll explain a little bit.
>
> Consider (show (read "13")).  The compiler has no way to know what the type "a"
> produced by read should be.  It must be an instance of (Read a) and (Show a),
> but the compiler cannot generate any actual code!
>
>
> Alfonso Acosta wrote:
> > Hi all,
> >
> > I have a type problem in my code which I dont know how to solve (and
> > I'm not really sure why is caused). I have made a simplified example,
> > can anyone have a look at it?
> >
> > Thanks in advance,
> >
> > The error is:
> >
> > Example.hs:24:47:
> >    Ambiguous type variable `a' in the constraint:
> >      `HDPrimType a'
> >        arising from use of `supplySig' at Example.hs:24:47-55
> >    Probable fix: add a type signature that fixes these type variable(s)
> >
> >
> > Here is the code
>
> I have further simplified the example, removing and reordering parameters.
>
> >
> > ---
> > module Example where
> >
> >
> > data HDSignal a = HDSignal
>
> The above lets you use constructor HDSignal to create type (HDSignal a) for any
> type "a".
>
> > class HDPrimType a where
> > class PortIndex a where
> >
> > class SourcePort s where
> > -- Plug an external signal to the port
> > plugSig  :: (HDPrimType a) => s  -> (HDSignal a -> b) -> b
>
> The above is odd, in that the plugSig can internally produce (HDPrimType a =>
> HDSignal a) for any "a" that satisfies the constraint.
>
> Perhaps you want to use a functional dependency?
>
> > class SourcePort s a | s -> a where
>
> Or perhaps you need an existential forall ?
>
> > plugSig  ::  s  -> (forall a. HDPrimType a=> HDSignal a -> b) -> b
>
> > class DestPort d where
> > -- Supply a signal to the port
> > supplySig  :: (PortIndex ix, HDPrimType a) => ix -> d -> HDSignal a -> d
> >
>
> The above is odd, in that the DestPort can take (HDPrimType a => HDSignal a) for
> any "a" that satisfies the constraint.
>
> Perhaps you really want the type "d" in (DestPort d) to imply a specific type
> "a" with a functional dependency?
>
> > class DestPort d a | d -> a where
>
> Or perhaps you need a forall?
>
> > supplySig  :: (PortIndex ix) => ix -> d -> (forall. HDPrimType a => HDSignal a) -> d
>
> > -- Connect providing indexes
> > connectIx :: (SourcePort s, DestPort d, PortIndex dix) =>
> >              s -> dix -> d -> d
> > -- This can seem ugly,
> > -- it would be easier for us having different types for plugSig and
> > supplySig
> > -- but the final user would find it much more difficult to deal with
> > -- supplysig
> > connectIx s dix d = plugSig s (supplySig dix d)
>
> The above produced the error because the compiler has no clue what the type "a"
> is in the signatures for supplySig and plugSit.
>